Legislation on vehicle and heavy goods vehicle emissions stipulates, amongst other things, a reduction in the release of nitrogen oxides NO into the atmosphere. One known way to achieve this objective is to use the SCR (Selective Catalytic Reduction) process which enables the reduction of nitrogen oxides by injection of a reducing agent, generally ammonia, into the exhaust line. This ammonia may derive from the pyrolytic decomposition of an ammonia precursor solution, whose concentration may be the eutectic concentration. Such an ammonia precursor is generally a urea solution.
With the SCR process, the high levels of NOx produced in the engine during combustion at optimized efficiency are treated in a catalyst on exiting the engine. This treatment requires the use of the reducing agent at a precise concentration and of extreme quality. The solution is thus accurately metered and injected into the exhaust gas stream where it is hydrolysed before converting the nitrogen oxide (NOx) to nitrogen (N2) and water (H2O).
In order to do this, it is necessary to equip the vehicles with a tank containing an additive (generally urea) solution and also a device for metering the desired amount of additive and injecting it into the exhaust line.
Given that the aqueous urea solution generally used for this purpose (eutectic 32.5 wt % urea solution) freezes at −11° C., it is necessary to provide a heating device to liquefy the solution in order to be able to inject it into the exhaust line in the event of starting in freezing conditions.
Several systems have been provided in the prior art for this purpose. Generally, these systems comprise heating devices that involve either specific heating elements or a bypass of the engine cooling circuit (for example, see Application WO 2006/064001 in the name of the Applicant).
As regards the use of specific heating elements, it is known to put resistive heating elements inside the tank containing the urea solution, and optionally over the urea lines (feed lines and return lines, where appropriate), over the pump, the filter, etc. and other active components of the system.
Thus, for example, Application DE 10 2004 061 259 describes a system that uses separate resistive elements for the tank, pump, metering module and urea feed and return lines. This system uses temperature probes present in the tank, pump and metering module and when one of these probes indicates a temperature below a given threshold value (generally the freezing temperature of the solution), all these resistors are activated so as to thaw the whole of the system. Such a strategy has several drawbacks.
Firstly, when starting a vehicle in cold (freezing) weather, all of the electrical devices (for de-icing the glass panes, heating the passenger compartment, etc.) are generally started up simultaneously, which causes a peak in electric consumption to which that of heating the SCR system is then added.
Next, it should be noted that certain parts of an SCR system may sometimes contain the urea solution and sometimes not (cf., for example, the upper part of the tank and the lines in the case where the latter are purged). Therefore, the fact of heating these parts when they do not contain liquid needlessly consumes power and can also lead to the premature degradation of the resistive elements in these zones.